Optional parameters in explicitly implemented interfaces

Question

public interface IFoo
{
    void Foo(bool flag = true);
}

public class Test : IFoo
{
    void IFoo.Foo(bool flag = true) //here compiler generates a warning
    {

    }
}

The warning says that the given default value will be ignored as it is used in the context which does not allow it.

Why are optional parameters not allowed for explicitly implemented interfaces?

Why you need to override the default parameter that specified in IFoo? — gabba, Oct 04 '12 at 15:19

score 2 · Accepted Answer · answered Oct 04 '12 at 15:18

2

Explicitly implemented interface methods are always called with a target whose compile-time type is the interface, not an specific implementation. The compiler looks at the optional parameters declare by the method it "knows" it's invoking. How would you expect it to know to take the parameters from Test.Foo when it only knows of the target expression as type IFoo?

IFoo x = new Test();
x.Foo(); // How would the compiler know to look at the Test.Foo declaration here?

answered Oct 04 '12 at 15:18

Jon Skeet

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I didn't think it tryed to override it. I thought the implementation had to be declared similar to the interface declaration, word for word...I even didn't mean to override it. Now I see, this assumption was absolutely wrong...Now that warning does make sense. – horgh Oct 04 '12 at 15:27
You're not really "overriding" as such - you're *implementing*. – Jon Skeet Oct 04 '12 at 15:29
That's why I thought, compiler wouldn't think, I wanted to change the default value for the parameter – horgh Oct 04 '12 at 15:30

score 0 · Answer 2 · answered Oct 04 '12 at 15:19

0

I would use method overloading.

public interface IFoo
{
    void Foo(bool flag);
    void Foo();
}

public class Test : IFoo
{
    void Foo() {
        this.Foo(true);
    }

    void Foo(bool flag) {
         // Your stuff here.
    }
}

answered Oct 04 '12 at 15:19

Simon Germain

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Oops... :D I'll change my answer. – Simon Germain Oct 04 '12 at 15:19

Optional parameters in explicitly implemented interfaces

2 Answers2